Passive rfid tag and rfid system

ABSTRACT

A passive radio frequency identification (RFID) tag includes: a rectifier circuit that rectifies a signal obtained from an antenna and outputs the rectified signal as a DC voltage. A capacitor is connected to an output line of the rectifier circuit. A first regulator circuit generates a first regulator voltage by stabilizing the output DC voltage from the rectifier circuit. A control circuit starts operating when the first regulator voltage is applied, and the control circuit generates a control signal upon receipt of the modulation signal section of the wireless signal. A second regulator circuit generates a second regulator voltage by stabilizing the output DC voltage from the rectifier circuit in response to the control signal and outputs the second regulator voltage to the outside.

TECHNICAL FIELD

The present invention relates to a passive radio frequencyidentification (RFID) tag that operates with radio waves transmittedfrom a reader/writer as the power source, and an RFID system having thispassive RFID tag.

BACKGROUND ART

Passive RFIDs are a technique by which a tag on a secondary unitoperates with radio waves transmitted from a reader/writer of a primaryunit as a power source, and information is communicated between thereader/writer and the tag.

Conventional passive RFID tags include: a rectifier circuit thatrectifies an alternating current signal that is a signal received froman antenna that receives radio waves; a regulator circuit thatstabilizes an output direct current voltage from the rectifier circuit;a control circuit that operates according to an output voltage from theregulator circuit; and a demodulation/modulation circuit that isconnected to the antenna, that operates using the output direct currentvoltage from the rectifier circuit, and that undergoestransmission/reception control by the control circuit. A conventionalRFID tag having this configuration is disclosed in Japanese PatentApplication Laid-Open Publication No. 2007-122600, for example.

The antenna receives a transmission radio wave from a reader/writer, andthe received signal is supplied to the rectifier circuit as analternating current signal, and converted to a direct current voltage.The direct current voltage is supplied to the regulator circuit and thedemodulation/modulation circuit. The control circuit operated by theoutput voltage of the regulator circuit writes reception data to amemory, and reads data written to the memory and supplies the data astransmission data to the demodulation/modulation circuit. Thedemodulation/modulation circuit detects and demodulates the receivedsignal and supplies the received signal to the control circuit asreception data as well as modulating transmission data that is aresponse supplied from the control circuit to the antenna as thetransmission signal.

SUMMARY OF THE INVENTION

In recent years, usage methods have been realized in which, aside from ausage method in which a reader/writer writing information to the memoryof a tag of a secondary unit or reading in the written information, anexternal sensor is provided on the tag, and sensor information detectedby the sensor is read. However, if a sensor is provided externally,external circuits such as a sensor circuit are operated by the outputvoltage from the regulator circuit, and thus, the power consumption ofthe tag increases. As a result, it is difficult to ensure sufficientpower for a tag that relies on received radio waves as the power source,and the demodulation/modulation circuit cannot output a high intensitytransmission signal to be read by the reader/writer, resulting in thecommunication distance between the reader/writer and the tag beingshortened.

An object of the present invention is to provide a passive RFID tag andan RFID system by which it is possible to ensure sufficient power, evenif an externally connected circuit is provided.

A passive RFID tag according to the present invention is configured toreceive, through an antenna, a wireless signal that is transmitted froma reader/writer, and includes a non-modulation signal section and amodulation signal section that immediately follows the non-modulationsignal section, the passive RFID tag including: a rectifier circuit thatis connected to the antenna, and that is configured to rectify areception signal obtained from the antenna by reception of the wirelesssignal and output the rectified reception signal as a direct currentvoltage to a power source line; a capacitor that is connected to thepower source line, and to which the direct current voltage is applied; ademodulation/modulation circuit that is connected to the power sourceline, and that is configured to detect and demodulate the receptionsignal upon application thereto of the direct current voltage to obtainreception data, as well as to modulate transmission data and supply aresulting modulation signal to the antenna; a first regulator circuitthat is connected to the power source line, and that is configured toperform an operation of generating a first regulator voltage bystabilizing the direct current voltage; a control circuit that isconfigured to start operating by application thereto of the firstregulator voltage as a result of generation of the first regulatorvoltage by the first regulator circuit, and to generate a control signalupon receipt of the modulation signal section; and a second regulatorcircuit that is connected to the power source line, that is configuredto perform an operation of generating a second regulator voltage bystabilizing the direct current voltage in response to the controlsignal, and outputting the second regulator voltage to outside.

In the passive RFID tag according to the present invention, the controlcircuit is configured to generate an enable signal as the control signalwhen the demodulation/modulation circuit obtains the reception dataindicating an ON command, and the second regulator circuit is configuredto start an operation of generating the second regulator voltage inresponse to the enable signal.

Also, the passive RFID tag according to the present invention furtherincludes: a timer circuit that is connected to the power source line,and that is configured to measure a prescribed timer period when thevoltage of the power source line reaches the prescribed direct currentvoltage, wherein the timer circuit is configured to generate a firstenable signal after measuring the prescribed timer period, wherein thefirst regulator circuit is configured to start an operation ofgenerating the first regulator voltage in response to the first enablesignal, wherein the control circuit is configured to generate a secondenable signal as the control signal by application thereto of the firstregulator voltage as a result of generation of the first regulatorvoltage by the first regulator circuit, and wherein the second regulatorcircuit is configured to start an operation of generating the secondregulator voltage in response to the second enable signal.

An RFID system according to the present invention includes: areader/writer that is configured to transmit a wireless signal thatincludes a non-modulation signal section and a modulation signal sectionthat immediately follows the non-modulation signal section; and apassive RFID tag that is configured to receive, through an antenna, thewireless signal that is transmitted from the reader/writer, wherein thepassive RFID tag a rectifier circuit that is connected to the antenna,and that is configured to rectify a reception signal obtained from theantenna by reception of the wireless signal and output the rectifiedreception signal as a direct current voltage to a power source line; acapacitor that is connected to the power source line, and to which thedirect current voltage is applied; a demodulation/modulation circuitthat is connected to the power source line, and that is configured todetect and demodulate the reception signal upon application thereto ofthe direct current voltage to obtain reception data, as well as tomodulate transmission data and supply a resulting modulation signal tothe antenna; a first regulator circuit that is connected to the powersource line, and that is configured to perform an operation ofgenerating a first regulator voltage by stabilizing the direct currentvoltage; a control circuit that is configured to start operating byapplication thereto of the first regulator voltage as a result ofgeneration of the first regulator voltage by the first regulatorcircuit, and to generate a control signal upon receipt of the modulationsignal section; and a second regulator circuit that is connected to thepower source line, that is configured to perform an operation ofgenerating a second regulator voltage by stabilizing the direct currentvoltage in response to the control signal, and outputting the secondregulator voltage to outside. includes:

According to the passive RFID tag and the RFID system of the presentinvention, when providing an external connecting circuit, chargeaccumulates in the capacitor as a result of the output direct currentvoltage from the rectifier circuit when receiving the non-modulationsignal section of the wireless signal, and when receiving thenon-modulation signal section of the wireless signal, the secondregulator circuit outputs the second regulator voltage to the externalconnecting circuit, and in this case, the charge accumulated in thecapacitor is discharged, thus allowing for more than sufficient powersupply to the external connecting circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an RFID system having a passive RFID tag as Embodiment 1 ofthe present invention.

FIG. 2 is a waveform chart showing an operation of the passive RFID tagof FIG. 1.

FIG. 3 is a flow chart showing an operation of a control circuit in thepassive RFID tag of FIG. 1.

FIG. 4 shows an RFID system having a passive RFID tag as Embodiment 2 ofthe present invention.

FIG. 5 is a waveform chart showing an operation of the passive RFID tagof FIG. 4.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be explained in detail belowwith reference to the drawings.

FIG. 1 shows a circuit configuration of a passive radio frequencyidentification (RFID) tag 11 to which the present invention is appliedas Embodiment 1. The passive RFID tag 11 includes a rectifier circuit12, regulator circuits 13 and 14, a control circuit 15, and ademodulation/modulation circuit 16. The rectifier circuit 12 isconnected to an antenna 21. The antenna 21 receives a wireless signalthat is a radio wave transmitted from an antenna 32 of a reader/writer31, and supplies the wireless signal to the rectifier circuit 12 as areceived signal. The rectifier circuit 12 rectifies the received signal,which is the supplied alternating current signal, and outputs the signalas a direct current voltage. The direct current voltage output terminalof the rectifier circuit 12 is connected through a power source line PLto the regulator circuits 13 and 14 and the demodulation/modulationcircuit 16, and the output direct current voltage of the rectifiercircuit 12 is supplied to the regulator circuits 13 and 14 and thedemodulation/modulation circuit 16.

A capacitor 22 is connected to the power source line PL, which connectsthe direct current voltage output terminal of the rectifier circuit 12to the regulator circuits 13 and 14 and the demodulation/modulationcircuit 16. The capacitor 22 is connected between the power source linePL and ground, and is attached externally to an IC chip 20 thatconstitutes the tag 11. Charge accumulates in the capacitor 22 as aresult of the output direct current voltage from the rectifier circuit12 being applied thereto.

The regulator circuit 13 is a first regulator circuit that stabilizesthe output direct current voltage from the rectifier circuit 12 to apredetermined regulator voltage and outputs the regulator voltage to thecontrol circuit 15.

The regulator circuit 14 is a second regulator circuit that, similar tothe regulator circuit 13, stabilizes the output direct current voltagefrom the rectifier circuit 12 to a predetermined regulator voltage andoutputs the regulator voltage to an external connection terminal 23. Theexternal connection terminal 23 has connected thereto a microcomputercircuit 24 as shown in FIG. 1, for example. The microcomputer circuit 24is constituted of a computer, and operates using the regulator voltageoutputted from the external connection terminal 23 as a power sourcevoltage.

The control circuit 15 is connected to the regulator circuit 13 as wellas to the regulator circuit 14 and the demodulation/modulation circuit16. The control circuit 15 operates using the regulator voltageoutputted from the regulator circuit 13 as a power source voltage. Inembodiments of the invention, the control circuit 15 may include one ormore processors and memory, as well as supporting logic circuits toenable the control circuit 15 to generate control signals, discussedbelow, based on determining characteristics of an input signal.

The demodulation/modulation circuit 16 is connected to the antenna 21and operates using the output direct current voltage from the rectifiercircuit 12 as a power source voltage. The demodulation/modulationcircuit 16 detects and demodulates the received signal supplied from theantenna 21 and supplies the received signal to the control circuit asreception data as well as modulating transmission data that is aresponse supplied from the control circuit 15 to the antenna 21 as thetransmission signal. The demodulation/modulation circuit 16 may includeany appropriate circuit, such as a phase-locked loop (PLL) type circuitor other digital signal processor, for obtaining data and modulating acarrier wave with the data, or for receiving the modulated signal andextracting the data from the modulated signal.

The regulator voltages of the regulator circuits 13 and 14 may be thesame voltage or differing voltages, and are set by the operatingvoltages of the control circuit 15 and the microcomputer circuit 24.

The control circuit 15 obtains reception data from thedemodulation/modulation circuit 16 and generates transmission data as aresponse according to a command included in the reception data. Also,the control circuit 15 outputs an enable signal EN to the regulatorcircuit 14 according to the command in the reception data received fromthe demodulation/modulation circuit 16. The regulator circuit 14executes a voltage stabilization operation over the period during whichthe enable signal EN is supplied from the control circuit 15.

Next, the operation of the passive RFID tag 11 according to the statedconfiguration will be described according to the operation waveformchart of FIG. 2.

The wireless signal transmitted from the antenna 32 of the reader/writer31 includes a continuous wave (CW) signal section, a command modulationsignal section, and a modulation signal section as shown in FIG. 2. TheCW signal section is an unmodulated continuous wave, or in other words,the section for only the carrier signal, and is the portion indicatingthe preamble of the command. The command modulation signal section is asection for a signal obtained by modulating the carrier using a bitarray representing the command. The modulation signal section is asection in which the signal obtained by modulating the carrier using abit array representing another command or a parameter continues.Alternatively, the modulation signal section may be a signal obtained bymodulation using a specific bit array that does not represent a commandor a parameter. In the wireless signal shown in FIG. 2, the commandmodulation signal sections CM1 and CM3 include an ON command for theenable signal EN, and the command modulation signal section CM2 includesan OFF command for the enable signal EN.

When the CW signal section of the wireless signal is received by theantenna 21 at a time t1 as shown in FIG. 2, the received signal issupplied to the rectifier circuit 12 and rectified therein to form adirect current voltage. As shown in FIG. 2, after the CW signal sectionis received, the output direct current (DC) voltage from the rectifiercircuit 12 gradually rises while charging the capacitor 22, and reachesa substantially uniform DC voltage V1. This DC voltage V1 is supplied tothe regulator circuit 13 and the demodulation/modulation circuit 16, andactivates the regulator circuit 13 and the demodulation/modulationcircuit 16. The DC voltage is stabilized by the regulator circuit 13 andthe regulator voltage is generated and supplied to the control circuit15, and the control circuit 15 operates using the regulator voltage. Inother words, the passive RFID tag 11 is in a standby state by which itcan handle the modulation signal section including the commandmodulation signal section of the wireless signal. However, the powerconsumption of the control circuit 15 is low during the standby stateand the capacitor 22 can be sufficiently charged.

When the command modulation signal section CM1 is received by theantenna 21 at a time t2 following the reception of the CW signal sectionof the wireless signal, the received signal is supplied to the rectifiercircuit 12 and rectified therein to form the direct current voltage, andthus, as shown in FIG. 2, the output direct current voltage of therectifier circuit 12 continues to be the substantially constant directcurrent voltage V1.

The command modulation signal section CM1 of the wireless signal isdetected and demodulated by the demodulation/modulation circuit 16 andsupplied to the control circuit as reception data. As shown in FIG. 2,the command modulation signal section CM1 includes the ON command forthe enable signal EN, and thus, reception data indicating the ON commandfor the enable signal EN is supplied to the control circuit 15. As shownin FIG. 3, upon receiving the reception data, the control circuit 15determines whether or not the reception data is an ON command for theenable signal EN (step S101). If the reception data is the ON commandfor the enable signal EN, then the enable signal EN is generated asshown in FIG. 2 (step S102). The enable signal EN is supplied to theregulator circuit 14 and causes the regulator circuit 14 to operate. Theregulator circuit 14 receives the output DC voltage from the rectifiercircuit 12 and stabilizes the DC voltage, thereby generating theregulator voltage as shown in FIG. 2. The regulator voltage is suppliedthrough the external connection terminal 23 to the microcomputer circuit24, and thus, the microcomputer circuit 24 is started up by theregulator voltage.

When the modulation signal section is received by the antenna 21 at atime t3 following the reception of the command modulation signal sectionCM1 of the wireless signal, the received signal is supplied to therectifier circuit 12 and rectified therein to form the DC voltage, andthus, as shown in FIG. 2, the output DC voltage of the rectifier circuit12 continues to be the substantially constant DC voltage V1. The commandis not included in the reception data obtained from thedemodulation/modulation circuit 16, and thus, the control circuit 15continues generation of the enable signal EN. As a result, the regulatorcircuit 14 continues supplying the regulator voltage through theexternal connection terminal 23 to the microcomputer circuit 24, andthus, the microcomputer circuit 24 continues to operate using theregulator voltage.

When the command modulation signal section CM2 is received by theantenna 21 at a time t4 following the reception of the modulation signalsection of the wireless signal, the received signal is supplied to therectifier circuit 12 and rectified therein to form the DC voltage, andthus, as shown in FIG. 2, the output DC voltage of the rectifier circuit12 continues to be the substantially constant DC voltage V1.

The command modulation signal section CM2 of the wireless signal isdetected and demodulated by the demodulation/modulation circuit 16 andsupplied to the control circuit as reception data. As shown in FIG. 2,the command modulation signal section CM2 includes the OFF command forthe enable signal EN, and thus, reception data indicating the OFFcommand for the enable signal EN is supplied to the control circuit 15.As shown in FIG. 3, after execution of step S102, upon receiving thereception data, the control circuit 15 determines whether or not thereception data is an OFF command for the enable signal EN (step S103).If the reception data is the OFF command for the enable signal EN, thengeneration of the enable signal EN is stopped as shown in FIG. 2 (stepS104). The supply of the enable signal EN to the regulator circuit 14 isstopped, thereby causing the regulator circuit 14 to stop generating theregulator voltage. As a result, supply of the regulator circuit to themicrocomputer circuit 24 from the regulator circuit 14 is stopped,causing the microcomputer circuit 24 to stop operating.

If no wireless signal is received after the modulation signal section ofthe wireless signal, then as shown in FIG. 2, if, for example, amodulation signal section arrives at a time t5 after reception of thecommand modulation signal section CM3 indicating the ON command for theenable signal EN and the wireless signal is stopped at a time t6thereafter, the passive RFID tag 11 stops operating. Specifically, aslong as the modulation signal section is being received, the rectifiercircuit 12 outputs the DC voltage. As a result, the control circuit 15continues generating the enable signal EN, and the regulator circuit 14continues supplying the regulator voltage through the externalconnection terminal 23 to the microcomputer circuit 24. However, ifreception of the modulation signal section ends and the wireless signalitself ceases, then the rectifier circuit 12 no longer outputs the DCvoltage, and thus, after the time t6, the output DC voltage from therectifier circuit 12 is gradually reduced as shown in FIG. 2 as theaccumulated charge from the capacitor 22 is discharged. As a result, asshown in FIG. 2, the control circuit 15 stops generating the enablesignal EN, and as shown in FIG. 2, the regulator circuit 14 stopsgenerating the regulator voltage, and thus, the microcomputer circuit 24stops operating.

Thus, in Embodiment 1, during the period in which the ON command for theenable signal EN is being received and the enable signal EN is beingsupplied by the control circuit 15 to the regulator circuit 14, it ispossible to operate the external microcomputer circuit 24 in addition tooperating the various circuits within the passive RFID tag 11. In thiscase, the charge accumulated in the capacitor 22 is discharged, allowingfor more than sufficient power supply. Thus, communication between thereader/writer 31 and the passive RFID tag 11 can be achieved for alonger period of time than in conventional configurations, without aloss of power. Also, during operation of the microcomputer circuit 24,the charge accumulated in the capacitor 22 is discharged, and thus, evenif received power is temporarily reduced due to an obstacle between thereader/writer 31 and the passive RFID tag 11, the accumulated charge inthe capacitor 22 can compensate for the reduced power being received.

During operation of the microcomputer circuit 24, sensor measurementscan be obtained by sensors such as a temperature sensor, the measurementdata can be supplied to the control circuit 15, and the control circuit15 can store the measurement data in a memory (not shown). Thereader/writer 31 transmits a read command for the sensor measurement atan appropriate timing through a wireless signal transmitted through theantenna 32 to the passive RFID tag 11. In the passive RFID tag 11, whenthe control circuit 15 recognizes the read command for the sensormeasurement in the reception data obtained by thedemodulation/modulation circuit 16 through the antenna 21, the sensormeasurement is read from the memory, the control circuit 15 createstransmission data including the sensor measurement as a response, andthe demodulation/modulation circuit 16 modulates the transmission datasupplied from the control circuit 15 as the response and supplies themodulated transmission data to the antenna 21 as the transmissionsignal. Thus, it is possible for the reader/writer 31 to obtain sensormeasurements by receiving the transmission signal through the antenna32.

With the reader/writer 31, it is possible to ascertain the approximatedistance between the reader/writer 31 and the passive RFID tag 11through the modulation signal level returned from the passive RFID tag11. Also, if the reader/writer 31 accesses the same RFID tag 11 aplurality of times, or if access is performed over a plurality of timeswhile detecting the returned modulation signal level, then the datacollection time can be optimized by managing the ON/OFF time of thestart-up command of the second regulator circuit 14 on the basis of thecharge time calculated according to the ascertained distance.

FIG. 4 shows a circuit configuration of a passive RFID tag 51 to whichthe present invention is applied as Embodiment 2. In this passive RFIDtag 51, components that are the same as those of Embodiment 1 areassigned the same reference characters. Aside from the fact that theregulator circuits 13 and 14 are operated by the supply of enablesignals EN1 and EN2, the rectifier circuit 12, the regulator circuits 13and 14, the control circuit 15, the demodulation/modulation circuit 16,and the capacitor 22 are similar to those of the tag 11 of Embodiment 1.The capacitor 22 is attached externally to an IC chip 50 thatconstitutes the tag 51. The passive RFID tag 51 further has a timercircuit 52. The timer circuit 52 is connected to the DC voltage outputterminal of the rectifier circuit 12, or in other words, to the powersource line PL, and measures a prescribed timer period from the time atwhich the DC voltage from the rectifier circuit 12 reaches V1. The timercircuit 52 generates the enable signal EN1 as of when measurement duringthe prescribed timer period ends, and supplies the enable signal EN1 tothe regulator circuit 13. The prescribed timer period corresponds to thetime from when the DC voltage reaches V1 to when the wireless signaltransitions from the CW signal section to the modulation signal section,or immediately prior to transitioning to the modulation signal section.

The control circuit 15 operates with the regulator voltage outputtedfrom the regulator circuit 13 as a power source voltage, generates theenable signal EN2 after a prescribed delay time elapses from the startof the operation, and supplies the enable signal EN2 to the regulatorcircuit 14. The prescribed delay time may be a delay from when theregulator voltage is received to when operation of the control circuit15 stabilizes.

Other components are similar to those of Embodiment 1, and therefore,and descriptions thereof are omitted here.

Next, the operation of the passive RFID tag 51 of Embodiment 2 accordingto the stated configuration will be described according to the operationwaveform chart of FIG. 5.

The wireless signal transmitted from the antenna 32 of the reader/writer31 includes a CW signal section and a modulation signal section as shownin FIG. 5. The CW signal section is an unmodulated continuous wave, orin other words, the section for only the carrier signal, and is theportion indicating the preamble of the command. The length of the CWsignal section is constant. The modulation signal section is a sectionin which the signal obtained by modulating the carrier using a bit arrayrepresenting a command or a parameter continues. Alternatively, themodulation signal section may be a signal obtained by modulation using aspecific bit array that does not represent a command or a parameter.

When the CW signal section of the wireless signal is received by theantenna 21 at a time t1 as shown in FIG. 5, the received signal issupplied to the rectifier circuit 12 and rectified therein to form a DCvoltage. As shown in FIG. 5, after the CW signal section is received,the output DC voltage from the rectifier circuit 12 gradually riseswhile charging the capacitor 22, and reaches a substantially uniform DCvoltage V1. The DC voltage V1 is supplied to the timer circuit 52, andas shown in FIG. 5, the timer circuit 52 starts measurement for theprescribed timer period T1 at a time t2. During measurement performedduring the timer period T1, the regulator circuit 13 and the controlcircuit 15 do not operate, and thus, it is possible to charge thecapacitor 22 even faster.

As shown in FIG. 5, the timer circuit 52 generates the enable signal EN1as of when measurement during the prescribed timer period T1 is ended ata time t3, and supplies the enable signal EN1 to the regulator circuit13. The time t3 is a time immediately prior to a time t4 when thewireless signal transitions from the CW signal section to the modulationsignal section.

The output DC voltage V1 from the rectifier circuit 12 is stabilized bythe regulator circuit 13 in response to the enable signal EN1, and asshown in FIG. 5, the regulator voltage is generated and supplied to thecontrol circuit 15. The control circuit 15 is started up by theregulator voltage.

When the control circuit 15 starts operating, then as shown in FIG. 5,the enable signal EN2 is generated after a prescribed delay time haselapsed from the start of operation. The enable signal EN2 is suppliedto the regulator circuit 14, and causes the regulator circuit 14 tooperate. The regulator circuit 14 receives the output DC voltage fromthe rectifier circuit 12 and stabilizes the DC voltage, therebygenerating the regulator voltage as shown in FIG. 5. The regulatorvoltage is supplied through the external connection terminal 23 to themicrocomputer circuit 24, and thus, the microcomputer circuit 24 isstarted up by the regulator voltage. During the period when themodulation signal section of the wireless signal is arriving, theregulator circuit 14 continues supplying the regulator voltage throughthe external connection terminal 23 to the microcomputer circuit 24, andthus, the microcomputer circuit 24 continues to operate by the regulatorvoltage.

However, if reception of the modulation signal section ends and thewireless signal itself ceases during a time t5, then the rectifiercircuit 12 no longer outputs the DC voltage, and thus, the output DCvoltage from the rectifier circuit 12 is gradually reduced as shown inFIG. 5 as the accumulated charge from the capacitor 22 is discharged. Asa result, generation of the regulator voltage from the regulatorcircuits 13 and 14 is stopped, causing the microcomputer circuit 24 tostop operating.

Thus, in Embodiment 2, while receiving the CW signal section of thewireless signal, the capacitor 22 is sufficiently charged, both theregulator circuit 13 and the control circuit 15 are started upimmediately before the start of the modulation signal section of thewireless signal, the enable signal EN2 is supplied from the controlcircuit 15 to the regulator circuit 14, and the regulator voltage issupplied from the regulator circuit 14 to the microcomputer circuit 24.Thus, during the period in which the regulator voltage is being suppliedfrom the regulator circuit 14 to the microcomputer circuit 24, it ispossible to operate the external microcomputer circuit 24 in addition tooperating the various circuits within the passive RFID tag 51. In thiscase, the charge accumulated in the capacitor 22 is discharged, allowingfor more than sufficient power supply to the external microcomputercircuit 24. Thus, communication between the reader/writer 31 and thepassive RFID tag 51 can be achieved for a longer period of time than inconventional configurations, without a loss of power. Also, duringoperation of the microcomputer circuit 24, the charge accumulated in thecapacitor 22 is discharged, and thus, even if received power istemporarily reduced due to an obstacle between the reader/writer 31 andthe passive RFID tag 51, the accumulated charge in the capacitor 22 cancompensate for the reduced power being received.

Like Embodiment 1, in Embodiment 2, during operation of themicrocomputer circuit 24, sensor measurements can be obtained by sensorssuch as a temperature sensor, the measurement data can be supplied tothe control circuit 15, and the control circuit 15 can store themeasurement data in a memory (not shown). The reader/writer 31 transmitsa read command for the sensor measurement at an appropriate timingthrough a wireless signal transmitted through the antenna 32 to thepassive RFID tag 11. In the passive RFID tag 11, when the controlcircuit 15 recognizes the read command for the sensor measurement in thereception data obtained by the demodulation/modulation circuit 16through the antenna 21, the sensor measurement is read from the memory,the control circuit 15 creates transmission data including the sensormeasurement as a response, and the demodulation/modulation circuit 16modulates the transmission data supplied from the control circuit 15 asthe response and supplies the modulated transmission data to the antenna21 as the transmission signal. Thus, it is possible for thereader/writer 31 to obtain sensor measurements by receiving thetransmission signal through the antenna 32.

In Embodiment 2, the timer circuit 52 that measures the prescribed timerperiod T1 is provided, but alternatively, a configuration may be adoptedin which a voltage detector (or a voltage comparator) is provided so asto detect when the output DC voltage from the rectifier circuit 12 hasreached a prescribed voltage, and the first regulator circuit 13 isoperated.

In Embodiments 1 and 2, the example of the passive RFID tag wasdescribed, but a configuration can be adopted in which an external powersource is provided in the tag, and power from the external power sourceis used only when the tag is accessed by the reader/writer 31. By usingsuch a configuration, power consumption during the standby period can bereduced, thereby enabling low power consumption.

What is claimed is:
 1. A passive radio frequency identification (RFID)tag, comprising: a rectifier circuit connected to an antenna andconfigured to rectify a reception signal obtained from the antenna basedon the antenna receiving a wireless signal, the rectifier circuitfurther configured to output a rectified reception signal as a directcurrent (DC) voltage to a power source line; a capacitor connected tothe power source line, so as to have the DC voltage applied to thecapacitor; a demodulation/modulation circuit connected to the powersource line and configured to detect and demodulate the reception signalupon application thereto of the DC voltage to obtain reception data, andfurther configured to modulate transmission data and supply a resultingmodulation signal to the antenna; a first regulator circuit connected tothe power source line and configured to generate a first regulatorvoltage by stabilizing the DC voltage; a control circuit configured tobegin operating based on receiving the first regulator voltage from thefirst regulator circuit, and configured to generate a control signalupon receipt of a modulation signal section of the wireless signal, themodulation signal section following a non-modulation signal section ofthe wireless signal; and a second regulator circuit connected to thepower source line and configured to generate a second regulator voltageby stabilizing the DC voltage in response to the control signal, andfurther configured to output the second regulator voltage to outside thepassive RFID tag.
 2. The passive RFID tag according to claim 1, whereinthe control circuit is configured to generate an enable signal as thecontrol signal when the demodulation/modulation circuit obtains thereception data indicating an ON command, and wherein the secondregulator circuit is configured to start an operation of generating thesecond regulator voltage in response to the enable signal.
 3. Thepassive RFID tag according to claim 2, wherein the control circuit isconfigured to stop the enable signal as the control signal when thedemodulation/modulation circuit obtains the reception data indicating anOFF command, and wherein the second regulator circuit is configured tostop the operation of generating the second regulator voltage inresponse to the stopping of the enable signal.
 4. The passive RFID tagaccording to claim 2, wherein the ON command is included in themodulation signal section.
 5. The passive RFID tag according to claim 3,wherein the OFF command is included in the modulation signal section. 6.The passive RFID tag according to claim 1, further comprising: a timercircuit that is connected to the power source line, and that isconfigured to measure a prescribed timer period when the voltage of thepower source line reaches the prescribed DC voltage, wherein the timercircuit is configured to generate a first enable signal after measuringthe prescribed timer period, and wherein the first regulator circuit isconfigured to start an operation of generating the first regulatorvoltage in response to the first enable signal.
 7. The passive RFID tagaccording to claim 6, wherein the control circuit is configured togenerate a second enable signal as the control signal based on receivingthe first regulator voltage from the first regulator circuit, andwherein the second regulator circuit is configured to start an operationof generating the second regulator voltage in response to receiving thesecond enable signal.
 8. The passive RFID tag according to claim 7,wherein the timer circuit is configured to measure the prescribed timerperiod upon receipt by the rectifier circuit of a first reception signalcorresponding to the non-modulation signal section of the wirelesssignal and end measurement of the prescribed timer period immediatelyafter receipt by the rectifier circuit of a second reception signalcorresponding to the non-modulation signal section, and wherein thecontrol circuit is configured to generate the second enable signalimmediately after the rectifier circuit begins to receive a thirdreception signal corresponding to the modulation signal section of thewireless signal.
 9. A radio frequency identification (RFID) system,comprising: a reader/writer configured to transmit a wireless signalthat includes a non-modulation signal section and a modulation signalsection that immediately follows the non-modulation signal section; anda passive RFID tag configured to receive, through an antenna, thewireless signal that is transmitted from the reader/writer, wherein thepassive RFID tag includes: a rectifier circuit connected to the antennaand configured to rectify a reception signal obtained from the antennaby reception of the wireless signal and output the rectified receptionsignal as a DC voltage to a power source line; a capacitor connected tothe power source line, so as to have the DC voltage applied to thecapacitor; a demodulation/modulation circuit connected to the powersource line and configured to detect and demodulate the reception signalupon application thereto of the DC voltage to obtain reception data, andfurther configured to modulate transmission data and supply a resultingmodulation signal to the antenna; a first regulator circuit connected tothe power source line and configured to generate a first regulatorvoltage by stabilizing the DC voltage; a control circuit configured tobegin operating based on receiving the first regulator voltage from thefirst regulator circuit, and configured to generate a control signalupon receipt of the modulation signal section of the wireless signal;and a second regulator circuit connected to the power source line andconfigured to generate a second regulator voltage by stabilizing the DCvoltage in response to the control signal, and further configured tooutput the second regulator voltage to outside the passive RFID tag.